Of particular interest is the precise detection of moisture in gas mixtures, such as air, for example, using a small and inexpensive structure.
Conventionally, the following five methods are basically used for humidity sensing:    a.) Capacitive humidity measurement. Here a hygroscopic polymer film is used whose dielectric constant is changed by water absorption corresponding to the relative humidity. The thus changed capacitance of the capacitor is directly proportional to the relative humidity.    b.) Psychrometric humidity measurement. Psychrometers are devices which are equipped with a wet temperature sensor and a dry temperature sensor. Evaporation causes the wet sensor to cool down. The humidity can be ascertained by determining the temperature difference between the two sensors.    c.) Hygrometric humidity measurement. Hygrometric transducers are equipped with a material which expands or contracts depending on the humidity. Organic materials, plastics or porously sintered ceramic materials such as aluminum oxide or zinc oxide are used.    d.) Chilled mirror dewpoint hygrometer. With this very precise measuring method, the condensation of water vapor is evaluated by dewpoint discrimination. The temperature of a mirrored surface is reduced until it just begins to fog. The temperature measured at this moment corresponds to the dewpoint temperature.    e.) Laser-based humidity measurement. Laser-based humidity measurement uses the characteristic optical absorption of water vapor in the infrared region of the spectrum. Because of the optical path length required, this likewise very accurate method is of a size which is disadvantageous for many applications.
The disadvantage of conventional measurement systems of this kind is that they are either cost-intensive, e.g. as a result of a chilled mirror or laser optics, or do not achieve the required measuring accuracy for many applications.
Through the use of standard processes for producing semiconductor components (CMOS), conventional gas sensors based on suspended gate field effect transistors (SGFETs) have the potential to be very inexpensive to manufacture. CMOS stands for complementary metal oxide semiconductor. In addition, a sensor of this kind requires minimal electrical energy for its operation. The construction and method of operation are known e.g. from DE 19814857, DE 19956744 and DE 19849932. A large number of materials can be used for sensitive films of such gas sensors, making this technology a platform for manufacturing a large number of different gas sensors. For humidity measurement, e.g. polymers are used as the sensitive material, as disclosed in EP 1191332. The construction of an SGFET is shown schematically in cross-section in FIG. 1. Over the field effect transistor comprising a source electrode S and a drain electrode D, a gas sensitive film is disposed above an air gap. By absorption of the gas on a sensitive film, an initial change in potential is brought about which in turn causes the current through the transistor to change. Such a change in the current represents the sensor signal. Such a change in potential constitutes a first change in potential.
Conventional semiconductor devices are protected from environmental effects by hermetic sealing, e.g. by potting compounds. In the case of the gas sensors described here, by virtue of their operating principle, the part of the chip surface which together with the actual sensitive material constitutes the gas-sensitive part of the structure cannot be protected in this way. Due to the effect of environmental influences such as humidity and temperature, this unprotected surface is subject to processes resulting in undesirable instabilities in the sensor signal such as baseline drift or diminution of the gas signal. Another major difficulty is the requirement for selective detection of a gas component, i.e. in general the sensor signal is affected not only by the gas component to be detected but also by other gases. To reduce signal drift in GasFETs, the following approaches have been adopted.
The operating temperature is increased. By increasing the operating temperature, the unwanted effect of humidity and temperature can be reduced by stabilizing the operating temperature. Disadvantageously, due to the heating power necessary, the power consumption of the sensor is increased such that the advantage of low energy requirement is lost. For humidity sensors, an additional effect produced is that the relative humidity at the sensor reduces while the ambient humidity remains constant, which diminishes the measuring effect.
DE 10 2004 019 604 discloses an operating method for a GasFET which evaluates both the work function and the change in the capacitance of the sensitive film. This is used to improve the sensitivity of the gas sensor, specifically to reduce the sensitivity to humidity.